ENH: implement VTKViewer::setVtkRenderWindow

- This calls `VTKRenderer::setRenderWindow`. Needed to use a
vtkRenderWindow from another instance (example : for a QT
application, use QVTKWidget renderwindow)
- Rename `VTKViewer::getRenderWindow` to `getVtkRenderWindow`
- Use internal functions in `getVtkCamera` and `getVtkRenderer`